The two most prominent fan-made visual novels that I know of are Katawa Shoujo (put together by a 4chan group) and Everlasting Summer (put together by an equivalent Russian fan group). Both are very good games. If you look up info on the development of those games, or contact their creators, they might be able to give you some advice.

I think the Katawa Shoujo group more or less disbanded, but the Everlasting Summer group is still around and tinkering with their game; they patch their English language steam port pretty often, in any case.